1. Field of the Invention
This invention relates to a volume holographic memory and an optical information-recording/reproducing system using the volume holographic memory.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system utilizing principles of holography. The holographic memory system records and reproduces digital data in and from a memory medium formed of a photorefractive crystal such as lithium niobate (LiNbO3). The photorefractive effect utilized in the medium is a phenomenon in which electric charges generated by optical pumping move within the crystal to form a spatial electric field distribution, and the space charge distribution changes the refractive indexes of the crystal via a linear electro-optical effect, i.e. the Pockels effect. For example, in a ferroelectric crystal having the photorefractive properties, changed refractive indexes are caused in response even to a fine optical input pattern generally having 1000 lines or more per millimeter therein. Further, the photorefractive effect is generated in real time at a response speed on the order of microseconds to seconds in dependence on the material. Therefore, research has been carried out in various applications of the photorefractive crystal as a real-time holographic medium which does not require development of images. In the holographic memory system, it is possible not only to record and reproduce data in units of two-dimensional planar image pages but also to perform multiplexed recording by using a plurality of pages. A volume holographic memory enables three-dimensional recording by three-dimensionally using the above memory medium by transforming the same to a three-dimensional shape, such as a rectangular parallelepiped. In the volume holographic memory which is one type of Fourier transform hologram, two-dimensional image data is recorded page by page in a dispersed manner in the three-dimensional space of this recording medium. In the following, an outline of the volume holographic memory system will be described with reference to FIGS. 1A and 1B.
Referring first to FIG. 1A, an encoder 25 converts digital data to be recorded in a volume holographic memory 1 to a dot pattern image of light and dark on a plane, thereby arranging the data into a data array, for example, of 640 bits wide by 480 bits high to generate a unit page data sequence. This data sequence is sent to an SLM (Spatial Light Modulator) 12, implemented, for example, by a panel of a transmission TFT (Thin Film Transistor) liquid crystal display (hereinafter referred to as xe2x80x9cLCDxe2x80x9d).
The SLM 12 has modulating units of 640 pixels wide by 480 pixels high which correspond to a unit page. The SLM 12 modulates a light beam applied thereto to a signal beam having spatial optical ON/OFF signals in response to the unit page data sequence supplied from the encoder 25, and guides the modulated signal beam to a lens 13. More specifically, the SLM 12 allows the signal beam to pass therethrough where bits of the unit page data sequence which is an electric signal have a logical value xe2x80x9c1xe2x80x9d, and blocks the signal beam when bits of the unit page data sequence have a logical value xe2x80x9c0xe2x80x9d, whereby electro-optical conversion is achieved in response to each bit of unit page data, and a modulated signal beam indicative of the unit page data sequence is generated.
The signal beam enters the volume holographic memory 1 via a lens 13. The volume holographic memory 1 receives not only the signal beam, but also a reference light beam or reference beam incident thereon at an angle xcex2 (hereinafter referred to as xe2x80x9cthe incidence angle xcex2xe2x80x9d) with respect to a predetermined reference line orthogonal to the optical axis of the signal beam.
The signal beam and the reference beam interfere with each other within the volume holographic memory 1 as illustrated in FIG. 1B, and resultant interference fringes are stored as a refractive index grating in the volume holographic memory 1, whereby data recording is performed. Further, it is possible to apply the reference beam to the volume holographic memory 1 at different incidence angles xcex2 for angle-multiplexed recording of a plurality of two-dimensional plane data items, whereby three-dimensional data recording can be achieved.
When recorded data is reproduced from the volume holographic memory 1, only the reference beam is applied to the volume holographic memory 1 at the same incidence angle xcex2 as was set for recording the data, in a manner directed toward the center of an area in which the signal beam and the reference beam intersected with each other in recording the data. That is, differently from the case for recording, the signal beam is not applied to the volume holographic memory 1. Then, a diffracted light from the interference fringes recorded in the volume holographic memory 1 is guided to a CCD (Charge Coupled Device) 22 of the optical detector through a lens 21. The CCD 22 converts the light and shade of the incoming beam to electric signal pulses and supplies an analog electric signal having a level dependent on the luminance of the incoming beam to a decoder 26. The decoder 26 compares the analog signal with a predetermined amplitude value (slice level) and reproduces corresponding data represented by logical values xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d.
In the volume holographic memory, recording is performed in response to a two-dimensional plane data sequence as described above, so that it is possible to carry out angle-multiplexed recording by changing the incidence angle xcex2 of the reference beam. More specifically, application of the reference beam to the volume holographic memory 1 at various incidence angles xcex2 makes it possible to define a plurality of two-dimensional planes each serving as a recording unit within the volume holographic memory 1, thereby enabling three-dimensional recording. The technique of angle-multiplexed recording is disclosed in Japanese Unexamined Patent Publication Nos. Hei 2-142979 and Hei 10-97174.
It is an object of the present invention to provide an optical information recording/reproducing system which is capable of recording an interference pattern in a volume holographic memory at high density and permits downsizing of the system.
To attain the above object, the present invention provides an optical information recording/reproducing system comprising:
support means for removably supporting a volume holographic memory that is formed of a photorefractive crystal, and has a shape of a rotary member having a rotational symmetry axis, with plane surfaces perpendicular to the rotational symmetry axis formed at respective opposite ends thereof;
reference beam-irradiating means for irradiating the volume holographic memory with a coherent reference beam having a first wavelength;
signal beam-irradiating means for irradiating the volume holographic memory with a coherent signal beam which has the first wavelength and is modulated according to image data, to cause the signal beam to intersect with the reference beam within the volume holographic memory, thereby generating a refractive index grating of a three-dimensional optical interference pattern between the signal beam and the reference beam; and
means for detecting a diffracted light generated by irradiation of the reference beam on the refractive index grating of the optical interference pattern, wherein the support means holds the volume holographic memory in a position allowing the signal beam to pass through the plane surfaces of the volume holographic memory at the respective opposite ends thereof.
In one aspect of the optical information recording/reproducing system according to the present invention, said support means holds said volume holographic memory in a position where said rotational symmetry axis of said volume holographic memory is parallel with an optical path of said signal beam.
In another aspect of the optical information recording/reproducing system according to the present invention, said reference beam-irradiating means irradiates said reference beam on a side surface of said volume holographic memory.
In a further aspect of the optical information recording/reproducing system according to the present invention, said support means includes means for shifting said volume holographic memory along said rotational symmetry axis of said volume holographic memory.
In a still further aspect of the optical information recording/reproducing system according to the present invention, said support means includes means for rotating said volume holographic memory about said rotational symmetry axis of said volume holographic memory.
In one aspect of the optical information recording/reproducing system according to the present invention, said support means includes means for shifting said volume holographic memory along said rotational symmetry axis of said volume holographic memory, and rotating said volume holographic memory about said rotational symmetry axis.
In another aspect of the optical information recording/reproducing system according to the present invention, the system further comprises gating beam-irradiating means for irradiating said volume holographic memory with a gating beam having a second wavelength for increasing photosensitivity of said volume holographic memory and activating or deactivating said refractive index grating depending on whether said optical interference pattern is present or absent.
In a further aspect of the optical information recording/reproducing system according to the present invention, said gating beam-irradiating means irradiates said gating beam on a side surface of said volume holographic memory.
In a still further aspect of the optical information recording/reproducing system according to the present invention, said gating beam-irradiating means applies said gating beam in a manner restricted to a limited area within said volume holographic memory, where said signal beam and said reference beam intersect with each other.
In another aspect of the optical information recording/reproducing system according to the present invention, the system further comprises a compensation lens having a curved surface complementary to said side surface of said volume holographic memory via which said reference beam and said gating beam enter said volume holographic memory and a plane surface on a side thereof opposite to said curved surface, said compensation lens being arranged such that said complementary curved surface is spaced from said side surface of said volume holographic memory by a predetermined distance.
A volume holographic memory according to the present invention is formed of a photorefractive crystal that records a three-dimensional distribution of interference fringes generated by interference between a reference beam and a signal beam modulated according to image data, the volume holographic memory that is formed of a uniaxial crystal having a shape of a rotary member, and a rotational symmetry axis in parallel with an optical crystal axis of said uniaxial crystal, with plane surfaces perpendicular to said rotational symmetry axis formed at respective opposite ends thereof.
In one aspect of the volume holographic memory according to the present invention, said shape of a rotary member is a shape of a cylinder.
In another aspect of the volume holographic memory according to the present invention, said shape of a rotary member is a shape of a truncated cone.
An additional aspect of the invention provides an optical information recording system which comprises:
support means for removably supporting a volume holographic memory that is formed of a photorefractive crystal, and has a shape of a rotary member having a rotational symmetry axis, with plane surfaces perpendicular to said rotational symmetry axis formed at respective opposite ends thereof;
reference beam-irradiating means for irradiating said volume holographic memory with a coherent reference beam having a first wavelength; and
signal beam-irradiating means for irradiating said volume holographic memory with a coherent signal beam which has said first wavelength and is modulated according to image data, to cause said signal beam to intersect with said reference beam within said volume holographic memory and thereby generate a refractive index grating of a three-dimensional optical interference pattern between said signal beam and said reference beam;
wherein said support means holds said volume holographic memory in a position allowing said signal beam to pass through said plane surfaces of said volume holographic memory at said respective opposite ends thereof.